When it comes to carbon emissions, one of the greatest offenders is undergoing a change of heart. Concrete, the most widely used building material and the third-largest contributor of carbon dioxide in the United States, easily clinches the title as the building material with the greatest environmental impact. Manufacturers, representing a once-unlikely breed of eco-conscious individuals, are leading the way in improving the carbon footprint of their building products.
The measurable effects of such changes will vary greatly depending on the product’s ubiquity in the market. As a result, scientists and manufacturers are scrutinizing concrete with unprecedented interest. Even modest reductions in concrete’s carbon footprint will likely outweigh any major improvements made in the manufacturing of other building products such as recycled plastic moldings or low-VOC tackboard panels.
One of the most promising ideas, heavily explored by startup companies as well as universities, looks at modifying the concrete manufacturing process to reduce carbon-dioxide emissions. The methodologies generally fall into three camps of thought: reduce carbon-intensive ingredients in concrete mixtures, sequester carbon during production, and absorb atmospheric carbon dioxide during curing.
The first method—which is also the most developed in the marketplace—involves partially replacing the amount of Portland cement in concrete with less energy-intensive materials such as flyash and silica fume. The production of Portland cement, the true greenhouse gas culprit in concrete, contributes 5 percent of all manmade carbon emissions alone. In lieu of Portland cement, products like EcoSmart Concrete by the Canadian manufacturer of the same name use supplementary cementitious materials (SCMs) such as blast-furnace slag and rice husk ash to reduce concrete’s greenhouse gas contribution by as much as half.
Carbon sequestration, the second approach, offers even greater potential. California-based companies Calera and Carbon Sciences are seeking to capture the carbon dioxide emitted by power plants for use in cement manufacturing, therefore reducing the greenhouse gas emissions produced by both industries. The Carbonate Mineralization by Aqueous Precipitation (CMAP) method diverts preheated flue gas into pH-modified seawater, which precipitates the cementitious ingredients of calcium carbonate or magnesium carbonate as a result. Calera claims that for every ton of concrete manufactured with this process, up to half a ton of carbon dioxide—which equates to half the amount of emissions produced by conventional concrete production—is sequestered.
The third method takes advantage of the absorption potential inherent in concrete’s curing process. TecEco in Tasmania, Novacem in London, and CarbonCure in Nova Scotia, Canada, all manufacture concrete that absorbs atmospheric carbon dioxide as it hardens. Replacing the conventional lime admixture in concrete with magnesium silicate makes this feat possible. According to Novacem, about 0.6 tons of carbon dioxide—a slight improvement over the CMAP process, give or take—is captured for every ton of material, which absorbs its capacity in about a year.
With such significant benefits promised by each method, what could be the catch? For high SCM concrete, obtaining the right mix that both meets the performance requirements and reduces emissions can be a balancing act because the modifications can also prolong curing times by a few minutes or even by several days. Still in early development stages, carbon sequestering and absorbing concretes need further refinement; for example, magnesium silicate concrete needs to be porous to store carbon, but high porosity translates into low concrete strength. These new concrete mixtures will also require extensive testing and regulatory review—not to mention significant reworking of material and energy supply chains—before they can jump into the commercial market with both feet.
Nevertheless, with potential energy savings and heightened environmental awareness serving as powerful incentives, the industry is on its way to changing the track record of one of history’s oldest building materials.